This invention relates to a mold used for forming a contact lens and for a method of preventing the formation of small strands of contact lens material from the overflow of the contact lens mold during contact lens manufacture.
One way to manufacture soft contact lenses is to mold contact lenses in plastic molds. Typically there are two mold portions which when assembled form a cavity between the mold portions. A reactive mixture which reacts within the cavity forms a contact lens. Typically a first mold portion is dosed with the reactive mixture, and the second mold portion is placed on the first mold portion, and then the reactive mixture is reacted. The placement of the second mold portion onto the first mold portion typically causes excess reactive mixture to overflow the cavity contacting one or both mold portions. The mold portions commonly have flat flanges, and the excess reactive mixture commonly spreads out between the flanges of the two mold portions. The reaction of the reactive mixture is commonly radiation activated. The reactive mixture in the cavity reacts e.g. polymerizes and/or crosslinks to form the contact lens and the overflow reacts, to form an annular ring of polymer. In the typical manufacturing process the overflow area on the first mold portion (located on the bottom), which is the surface area of the first mold portion which the overflow reactive mixture will contact, receives an application of a surfactant prior to the dosing step. The surfactant prevents the overflow material from adhering to the first mold portion, and thereby helps the overflow material to adhere to the second mold portion and be removed from the manufacturing process when the second mold portion is removed from the first mold portion, referred to as the de-mold step. The second mold portion is then discarded. The contact lens stays in the first mold portion which continues in the contact lens manufacturing process to the hydration and washing step or steps. The hydration and washing step or steps have been disclosed in the prior art, including U.S. Pat. Nos. 5,640,980; and 5,690,866, incorporated herein by reference.
During the hydration and washing step(s), any residual reactive monomer overflow, which if present is usually a thin or small piece which broke off from the bigger piece (which adhered to the second mold portion) washes off the first mold portion flange and may attach itself to a contact lens, which may later cause the contact lens to be rejected.
Typically the dosing and reacting steps occur in an inert environment; however, recently it was discovered that contact lenses can be manufactured in an ambient environment as long as the time between dosing the reactive mixture into the first mold portion and the placement of the second mold portion onto the first mold portion is less than 70 seconds. The process of manufacturing contact lenses in an ambient environment was disclosed in U.S. patent Ser. No. 09/222,266, now U.S. Pat. No. 6,241,918, incorporated herein by reference. The reactive mixture which forms the contact lens within the mold is not effected by the oxygen in the ambient environment once the second mold portion is placed on the first mold portion, because within the closed mold, the reactive mixture is sealed away from the ambient environment; however, the overflow reactive mixture on the flange is exposed to the oxygen in the ambient environment which may interfere with and prevent the fall reaction, e.g. polymerization, of the reactive mixture. It is suspected that this may be causing a greater formation of thin pieces of the reactive mixture which do not react fully and often break away from the rest of the overflow material. The small pieces of the partially polymerized reactive mixture do not adhere to the second mold portion at the time the second mold portion is removed from the first mold portion during the de-mold step. Instead the small pieces tend to find their way to and stick to the contact lenses during the subsequent hydration and/or washing step(s).
This invention addresses the problem of the pieces of reactive mixture which break off from the rest of the overflow and/or are not removed with the second mold portion, and/or are not easily removable from the mold portions. This invention is useful for any contact lens manufacturing line, e.g. the reaction occurs in an inert or ambient environment.
This invention provides a mold for forming a contact lens comprising an overflow collector. The overflow collector is a volume preferably at least partly defined by a structure which causes the overflow reactive mixture to be accumulated, and not to spread out as it would upon the typically flat surface of a prior art mold. The overflow collector also reduces the amount of the surface area of the overflow that contacts the ambient or inert environment, preferably the overflow collector reduces the surface area of the overflow that contacts the ambient or inert environment by more than twenty-five percent as compared to a mold having a flat overflow area, more preferably by more than fifty percent as compared to a mold having a flat overflow area. The overflow collector assists in the removal of the overflow reactive mixture from the contact lens manufacturing process. The overflow collector preferably prevents the formation of thin strands of reactive mixture. This overflow collector preferably assists in the adhesion of the overflow reactive mixture to at least one of the mold portions.
This invention further provides a method of preventing the formation of contaminating pieces, typically thin strands of overflow reactive mixture comprising the step of: preventing the overflow reactive mixture from spreading out on the mold. The thin strands usually have dimensions between 40 to 1,000 microns. This method also reduces the amount of the surface area of the overflow that contacts the ambient or inert environment. This method assists in the removal of the overflow reactive mixture from the contact lens manufacturing process. This method preferably prevents the formation of thin strands of reactive mixture. This method preferably assists in the adhesion of the overflow reactive mixture to at least one of the mold portions.